Lift stations are used to help transport liquid wastewater from homes and businesses across the city to a treatment plant for processing and cleaning. Sewage from homes or buildings travels by gravity through sloped primary sewage pipes to a main collection well or holding pit. Such a well or holding pit, known as a wet well, is part of a lift station. Once the sewage arrives at the lift station a sewage pump actively pushes the sewage from the wet well to a force main for delivery to a treatment facility. The pump also may send the sewage to a gravity flow pipe positioned at a higher level than the wet well of the lift station. Lift stations are necessary as sewage cannot flow passively uphill. At some point the sewage must be either raised to a higher level for continued gravity flow or the sewage must be forcefully pumped through pipes to a higher level destination.
A frequent and significant problem with lift stations is odor generation from sewer gas. The presence of sewer gas raises concerns for sewer personnel exposed to accumulated gases. Also, sewer gases left to accumulate in air-tight environments can create additional toxic gases and underground potential for explosion, stagnation, and dead space in lines. Aeration is an option to reduce odor. Many odors accumulate because of oxygen-deficient environments. Sometimes chemicals such as sodium nitrate are added to increase oxygen content in the water to provide the bacteria an additional oxygen source when the oxygen in the sewage is exhausted. This prevents H2S formation from anaerobic bacteria. In some cases, pine scented blocks also have been added to neutralize the obnoxious odors.
A main characteristic of sewage or wastewater is its highly disagreeable odor. Many organic and inorganic compounds contribute to the formation of this odor. But mostly inorganic and organic sulfur compounds are responsible for the odor. The principal agent among the sulfur compounds is hydrogen sulfide. Hydrogen sulfide in sewage or wastewater creates a public nuisance because of its highly disagreeable odor at levels as low as 0.01 parts per million (ppm) in the atmosphere. Breathing by humans of air containing as low as 10 ppm of hydrogen sulfide for a period of time can prove fatal; such accumulations are likely to occur in covered or domed clarifiers or settling tanks in water treatment plants, or in manholes or wet wells in sewage systems. Besides odor, hydrogen sulfide formation results in heavy corrosion of metal and concrete sewer lines, concrete and metal structures, and metal equipment and machinery. The corrosion is greatly reduced when the formation of hydrogen sulfide can be reduced in sewage. The presence of hydrogen sulfide also is detrimental to the growth of microorganisms and other biological active substances. The microorganisms and biological active substances can not properly metabolize in the presence of hydrogen sulfide. Thus the efficiency of purification of sewage or wastewater is decreased.
The condition under which hydrogen sulfide is formed in sewage or wastewater is called septic. In septic sewer conditions the microorganisms present turn black and seems lifeless. Until this condition is reversed, sewage or wastewater can not be efficiently purified by biological active substances. Any added biologically active substances will not function properly in septic conditions and will fail to remove the nutrients properly from the sources in sewage.
Although, many treatments to control formation and regeneration of hydrogen sulfide in sewage or wastewater have been tried, none has been uniformly satisfactory or successful in both gravity systems and force main systems. Among those treatments used have been aeration, chlorination, ozonation, lime, sodium nitrate, activated carbon filtration, odor masking, hydrogen peroxide oxidation, and iron salts. However, the high cost and frequency and sometimes difficulty of adding these materials to a force main in particular make these options or their methods of addition to a force main unsatisfactory.
Therefore, it would be beneficial in terms of cost and efficiency if a means of treating force sewer mains were available that was inexpensive, required only limited operator involvement, did not require the addition of manufactured chemical products to the sewage, and would rid the sewage force mains of the hydrogen sulfide producing bacteria.